1. Field of the Invention
The present invention relates to an image signal compression encoding apparatus and to an image signal expansion reproducing apparatus, and in particular, to an image signal compression encoding apparatus in which after a two-dimensional orthogonal transformation of a video signal, the video signal can be normalized by use of a desired normalization coefficient and to an image signal expansion reproducing apparatus for decoding data thus encoded.
2. Description of the related Art
When loading a memory with digital image data such as image data shot by an electronic still camera, in order to reduce the amount of data to minimize the storage capacity of the memory, various kinds of compression and encoding operations are performed on the image data. Particularly, two-dimensional orthogonal encoding has been commonly employed because the encoding can be accomplished with a high compression rate and image distortion in the encoding can be suppressed.
In such a two-dimensional orthogonal transformation of a video signal, the image data is subdivided into a predetermined number of blocks. The transformation is conducted on image data of each block. Obtained image data, namely, a transformation coefficients are compared with a predetermined threshold value and the that portion does not exceed the threshold value is truncated (i.e. a coefficient truncation is achieved.) As a result, thereafter, any translation coefficients not exceeding the threshold are treated as data of zero in the processing. The transformation coefficients, which have undergone the truncation, are then divided by using a predetermined quantization step value, namely, a normalization coefficient to be quantized or normalized depending on the step width. Through the operation above, the value of the translation coefficients, namely, the dynamic range of the amplitude can be suppressed.
The normalized translation coefficients are thereafter encoded. By the way, the transformation coefficients include data items arranged from a low-frequency range to a high-frequency range depending on the magnitude of each block of the image data. Since the data of the normalized transformation coefficients become 0 in the high-frequency component, run-length encoding is achieved in which the original data are translated into a continuation length of value 0, namely, a so-called run length of 0 and a value of data including values of other than 0, namely, a so-called amplitude of non-zero. The resultant data is then subjected to a two-dimensional Huffman encoding to produce compressed image data thus encoded.
In the two-dimensional orthogonal transformation coding, by altering the value of the normalization coefficient, the image data can be encoded with various compression rates. For example, with a large value of the normalization coefficient, the normalized translation coefficient data takes a small value. Consequently, the compression rate of the image data is increased and the picture quality of the attained data is lowered. Conversely, for a small value of the normalization coefficient, the image data is compressed with a small compression rate and hence a high picture quality is developed for the attained data.
As a consequence, although the normalization is to be achieved with various kinds of normalization coefficients, normalization coefficient data used in an inverse normalization to be conducted by a reproducing apparatus is required to be changed. This leads to a problem that the normalization coefficients cannot be arbitrarily selected. In particular, after the orthogonal translation, when the compression rate of the encoding is desired to be varied between the low-frequency and high-frequency components, there arises a problem of a difficulty in setting the normalization coefficient to the different values.